1. FIELD OF INVENTION
The present invention relates to a high speed rotor assembly that is strongly required to stably rotate at a high accuracy, such as rotary bearing for polygon mirror.
2. RELATED ART
Polygon mirrors are used in, for example, laser printers, bar code readers, facsimiles and laser copy machines. Since the efficiency of the polygon mirror is enhanced as the number of rotations increases, the number of rotations of 20000 r.p.m. or more has been preferred . As the bearing for such a high speed rotor assembly, for example, precise ball bearings have heretofore been used. However, it is difficult to keep a stable accuracy of rotation for an extended period of time due to such problems as wearing and that the limit of the rotational speed is about 16000 r.p.m.
Accordingly, grooved pneumatic dynamic pressure bearings for bearing a rotor rotating at a high speed have been used. The grooved pneumatic dynamic pressure bearings have features of low noise and low friction loss since they bear a rotor via a gas film in a non-contacting manner. However, they are weak for external shocks and vibrations of the rotor are may occur since the load bearing ability of the bearing is low. Thus it is difficult to obtain a stable accuracy of rotation.
In order to overcome the above-mentioned problems, there has been proposed, inter alia, a high speed rotor assembly in which the bearing rigidity is increased to suppress the vibration of the rotor 52 by enlarging the bearing 51 as disclosed in Japanese Unexamined Patent Publication No. Tokkai Sho 60-98213 and shown in the sectional view of FIG. 8. Another proposed solution has been a high speed rotor assembly, as disclosed in Japanese Unexamined Patent Publication No. Tokkai Sho 63-266420 and as shown in the section view of FIG. 9, in which: thrust direction pneumatic dynamic pressure bearings 62a and 62b are provided at opposite ends of the radial pneumatic dynamic pressure bearing 61 to suppress the vibrations of the rotor 63 by the pneumatic dynamic pressure generated in a thrust direction; and the size of a rotary shaft 61 is reduced.
In these high speed rotor assemblies which use axially long pneumatic dynamic pressure bearing having an increased radial rigidity for bearing the rotor , the rotor is larger than the rotor in the high speed rotor assembly using conventional ball bearings, thereby resulting in an increase in the size of the assemblies. One the other hand, the pneumatic dynamic pressure bearings generally require precise machining in which the accuracy of shape such as degree of cylindricality, roundness and surface roughness should be in the order of 1 .mu.m to provide excellent accuracy of rotation. Therefore, the fact that the rotary shaft is long in an axial direction makes it difficult to perform machining. Although it is possible to perform machining, the manufacturing cost is so high that it is impractical.
Since the area of contact between the rotor and the thrust bearing surface is large in the rotor assembly in which the radial pneumatic dynamic pressure bearing is provided at both ends thereof with the thrust dynamic pressure bearings, a high starting torque is required for starting the rotor. As a result ringing effect (adsorption effect on the smooth face) mayoccur on the thrust bearing face which may prevent activation of the rotor assembly.
When the inner peripheral side of a cylindrical work piece is machined, the inner diameter at opposite ends tends be less or higher than that at the midposition thereof. In other words, the inner peripheral side of the work piece may be machined into arcuate shape as shown in FIGS. 2A and 2B. Therefore, unstable dynamic pressure distribution is liable to occur at the both ends of the radial bearing due to the changes in the gap of the bearing and the resulting changes in the number of rotations. Vibrations in a conical mode may occur due to occurrence of unforeseen vibrations, amplification of the vibrations, of the rotor at a specific vibration frequency of synchronized vibration and resonation of the vibration with the magnetic force balance in a thrust direction. As a result, the bearing of the rotor may contact with the stationary shaft and the vibrations due to shock may not be dumped.
Therefore, the present invention was made in order to overcome the above-mentioned problems. It is an object of the present invention to provide a high speed rotor assembly which is compact in size; requires a less starting torque; and provides a superior stable accuracy of rotation in a wide range from the beginning of rotation to a high speed rotation to a high shock environment.